Various current communication systems utilize passive optical network (PON) technology. Network operators presently utilize PONs to provide broadband communications services, such as data, subscription television and telephony, to homes and small businesses. Such PON systems typically can support a maximum optical fiber reach of 20 km (i.e., from the central office to the subscriber), and a maximum “split ratio” of 32 subscribers per feeder fiber. These limits are due to limitations in optical transmitter power output and optical receiver sensitivity in commercially available components. One way to extend the reach and increase the split ratio of a PON is to use optical amplifiers to compensate for the additional fiber and optical splitter losses. It is noted that the invention discussed below focuses on Gigabit-capable PON (GPON), ITU-T Recommendation G.984. However, it is also applicable to other PON technologies, including but not limited to, Broadband PON (BPON, ITU-T Recommendation G.983) and Gigabit Ethernet PON (GEPON, IEEE 802.3ah).
Existing PONs typically operate on a wavelength plan of approximately 1490 nm in the downstream direction, and 1310 nm in the upstream direction. In order to use the same wavelength band for extended range and/or larger split ratio, semiconductor optical amplifiers (SOAs) are presently a primary cost effective technology which can be designed for use in 1490 nm and 1310 nm wavelength band.
An SOA typically has an approximately 40 nm useable waveband. The standardized downstream GPON waveband is in the range of 1480 nm to 1500 nm, or about 20 nm wide. For upstream transmission, the current GPON standards specify a waveband of 1260 nm to 1360 nm, or about 100 nm wide. Typical upstream lasers actually operate at around 1310 nm, with a waveband which is about 20 nm to 30 nm wide. In order to reduce the SOA's amplified spontaneous emission (ASE) noise contribution to the upstream signal quality, the upstream signal band should be limited to about 20 nm, such as from 1300 nm to 1320 nm, e.g., using a coarse wavelength division multiplexed (CWDM) laser.
FIG. 1 illustrates a typical amplified PON system 10. Referring to FIG. 1, the system includes an optical network unit (ONU) 12, a 1×N optical coupler 14 (as a variation, 2×N optical couplers are utilized in protected PON designs), a first wavelength division multiplexer (WDM) 16 and a second wavelength division multiplexer 18, which are coupled to a first SOA 20 and a second SOA 22. In the given embodiment, the first SOA 20 amplifies signals propagating in the downstream direction, and the second SOA 22 amplifies signals propagating in the upstream direction. The system 10 further includes an optical line terminator OLT, which is located in the central office. As shown, the OLT includes a transmitter 26, a receiver 28, and a WDM 32, which couples both the transmitter 26 and the receiver 28 to the feeder fiber.
With respect to the operation, when an ONU 12 has data to send, and further has received a transmission grant as defined in the PON protocol, the ONU 12 sends a burst of data in the upstream direction, through one (or more) SOA 22 to the OLT in the central office. The amplified PON 10 has a plurality of ONUs 12 coupled to the first SOA 22 and feeder fiber by the N-port optical coupler 14. Thus, in the upstream direction, the coupler 14 combines the output signals from the ONUs 12, and couples the combined signal to the input of the first upstream SOA 22, by way of the WDM filter 16. The received power level at the upstream SOA 22 may vary between ONUs 12, due, e.g., to differences in the lengths of distribution fibers and to variations in ONU transmitter output power. Thus, the upstream input signal at the SOA 22 will have wide dynamic range over timescales of the order of 1 μs to 10s of μs or more.
Optical amplifiers, such as SOAs, are typically designed to be either constant gain or constant power amplifiers. In the PON application, the downstream SOA 20 may be either constant gain or constant power. However, for the upstream SOA 22, constant gain operation is necessary due to differences in input signal level from the different ONUs 12, and the use of burst mode operation for transmitting data in the upstream direction.
It is well known that semiconductor devices experience permanent changes in their crystalline structure over time and use, which affects their performance. Due to this aging effect, SOA gain will drift over time. Thus, proper means to monitor and compensate for gain variation must be implemented. However, measurement of gain of upstream SOA 22 with sufficient accuracy to monitor and compensate for aging is made difficult by the dynamic range of the upstream input signal.
Accordingly, there is a need for an effective, cost sensitive method and apparatus for controlling the gain of SOA's utilized in the upstream direction in PON applications, which at the same time could provide for an optical supervisory channel for communication between the central office (CO) and a remote node. It is an object of the present invention to provide a method and apparatus which achieves these objectives.